Admiral Aaron is a senior officer at Starfleet Headquarters who is controlled by one of the neural parasites that infiltrate the command structure in 2364.
Aaron appears in Conspiracy. When Picard brings the Enterprise-D to Earth to investigate the warnings of his murdered friend Walker Keel, Aaron is one of three admirals, with Savar and Gregory Quinn, who welcome him to Starfleet Headquarters.
All three are hosts to the small parasitic creatures that have been quietly taking over key officers. Aaron attends the dinner at which Picard is served a bowl of live grubs and realises how far the infiltration has gone.
When Riker, who has only pretended to be infected, opens fire, Aaron flees the room. Picard and Riker pursue and stun him in the corridor, and the parasite leaves his body and leads them to the mother creature inside Dexter Remmick.